Fuel cells, expected as a bargaining chip to solve energy and environmental issues, have been positively developed in recent years. Especially, a fuel cell with a solid polymer film used as its electrolyte is easy to handle owing to its low operating temperature of approximately 80° C., and thus is currently a mainstream in development of fuel cells. However, in such a type of fuel cell, where hydrogen is used as a fuel, a gas sensor for sensing hydrogen is required as a safety measure against leakage.
A conventional gas sensor senses the change in heat conductivity due to the presence of hydrogen, as the change in temperature of a heater element, using a property of hydrogen that is its heat conductivity is extremely high compared to other gases. For example, if hydrogen exists in the air, more heat is lost from the heater element than in a case where hydrogen does not exist. Consequently, the temperature of the heater element changes according to the hydrogen concentration. The conventional gas sensor electrically senses this temperature change as the change in resistance value of the temperature-sensing element.
The gas sensor uses a platinum thin-film resistive element for the heater element (also used for a temperature-sensing element). This element, a thin-film, can be manufactured using semiconductor fine processing technology (i.e. micromachining), enabling a submicroscopic heater element to be formed. Accordingly, the response speed of the gas sensor is enhanced with lower power consumption. Such a gas sensor is disclosed in Japanese Patent Unexamined Publication No. 8-101156, for example.
When such a gas sensor is used to sense hydrogen leakage, moisture in hydrogen, which is a gas to be sensed, causes a problem. That is, if moisture does not exist, the resistance value of the heater element changes according to the hydrogen concentration. Meanwhile, if moisture exists, the resistance value changes according to the humidity also. Therefore, it is not possible to distinguish the change if it is due to hydrogen and/or moisture.
Consequently, the conventional gas sensor changes the current passing through the heater element. This changes the output voltage across the heater element according to the degree of response. Then, the voltages across the heater element when different amounts of current are passed are substituted into an estimation equation obtained in advance, and then simultaneous equations are produced. Accordingly, the amount of each gas, namely the concentration of each gas is obtained from the solutions to the estimation equations.
Basically, such a solving method allows the gas concentrations of a plurality of components to be obtained. However, a problem exists where, at approximately 80° C., in such as sensing leakage from a fuel cell, hydrogen almost saturated with vapor leaks into the atmospheric air. If the change in heat conductivity of each of gas components is expressed with a linear expression as mentioned above, or if sensing is used only for a range where a linear expression well expresses, hydrogen concentration and humidity can be obtained using Chebyshev orthogonal polynomial. However, in the above-mentioned case, a large amount of vapor is assumed to exist as compared to hydrogen. Such a case shows a nonlinear (always having an order of two or more) characteristic. That is, the heat conductivity of the mixture once rises along with the humidity to a peak, and then falls. Therefore, just solving estimation equations simultaneously leads to troublesome calculation. Further, a plurality of solutions exist for humidity, and the humidity cannot be determined uniquely, thus neither can be the hydrogen concentration.